This invention relates to the field of automotive hydraulic systems, and more particularly to automotive hydraulic systems having a power steering system which supplies power to another hydraulically powered device.
Various types of hydraulically powered accessories are commercially available for trucks, vans, and other automotive vehicles, including snow plows, liftgates and dump beds. In the case of liftgates, for example, all known present designs have two things in common: A hydraulic cylinder for motion, and a 12-volt self-contained hydraulic unit to power the cylinder. 12-volt self-contained pumps, although easily connected to a source of power in the vehicle, are unreliable, hard to maintain, have virtually no cold weather operability, and are one of the most expensive components of the lift.
It has been suggested in the past to supply hydraulic fluid under pressure from the power steering pump of a vehicle to certain hydraulic devices, e.g., a cooling fan in U.S. Pat. No. 5,535,845 to Buschur and a brake power booster in U.S. Pat. No. 5,549,361 to Sorensen.
The Sorensen system has a brake power booster and two series-connected solenoid valves connected in parallel with a power steering valve. A boost developing valve is added in the line from the hydraulic pump to the power steering valve to provide for transfer of fluid flow from a normally open one-way path to a one-way flow-restrictive path in response to a force being applied to the brake pedal. The system appears to be designed to maintain an essentially constant pressure for the brake booster regardless of the pressure or other requirements of the rest of the system. While such a system may be suitable for a brake booster, which is typically small enough to be filled with hydraulic fluid in substantially less than one second at a typical flow rate of about 4 gallons per minute (GPM) delivered by a power steering pump, it could cause a loss of power steering for several seconds if used in conjunction with an external device requiring larger volumes of fluid. Such a sustained loss of power steering would be especially likely if the device were operated at no load, because in that situation virtually all of the fluid would be diverted to the external device. The Sorenson patent contains no apparent recognition of such a potential problem or of the desirability of driving a relatively high-volume device in parallel with a power steering unit.
Buschur suggests a series combination of an automotive fan motor and a power steering unit and employs a bypass line around the fan motor to give priority to operation of the power steering unit, i.e., to halt the operation of the fan if the load on the pump becomes excessive.
A hydraulic winch powered by a vehicle's power steering pump is commercially available from MileMarker of Pompano Beach, Fla. The winch and the power steering unit are connected in series in the MileMarker system via a dual-solenoid valve with four ports. Flow from the pump to the steering gear box is rerouted through the valve to the winch, and back through the valve to the steering gearbox so that fluid is always flowing through the steering gearbox. A hand control with a connection to each solenoid is included.
There remains a need for valving systems that eliminate many recurrent problems and design limitations in connection with hydraulic accessories such as liftgates, snow plows, and dump beds.